1. Field of the Invention
The present invention relates to a carbon electrode. More particularly, the present invention is concerned with a carbon electrode not only having excellent mechanical strength but also being chemically stable so that even when the carbon electrode is used as an anode in the electrolysis of an HF-containing molten salt (in this electrolysis the carbon electrode is exposed to a fluorine atmosphere entraining HF and therefore is likely to form an intercalation compound with fluorine and hydrogen fluoride, which has for the first time been found by the present inventors to be a cause of cracking of a carbon electrode), the carbon electrode is substantially free from the danger of breakage or cracking during the electrolysis. The carbon electrode of the present invention can advantageously be utilized not only for stably conducting the electrolysis of an HF-containing molten salt but also for obtaining an electrolysis product of high purity. The present invention is also concerned with a method and an apparatus for the electrolysis of a hydrogen fluoride (HF)-containing molten salt by the use of this carbon electrode as an anode.
2. Discussion of Related Art
As a representative example of electrolysis of an HF-containing molten salt, electrolytic production of fluorine can be mentioned. As a method for producing fluorine, the so-called middle temperature method, in which the electrolysis of a molten salt composed of KF and HF is conducted at about 90.degree. C., is generally employed.
In the case of the middle temperature method, KF-2HF is widely used as the composition for a molten salt electrolytic bath since, with this composition, the vapor pressure of HF is low at a temperature around the melting point of the molten salt and, in addition, the melting point of the molten salt is substantially not affected by a change in the HF concentration of the bath. As the material for the anode of the electrolytic cell, carbon is mainly employed since a metal cannot be used due to the danger of melting of a metallic anode during the electrolysis. As the material for the cathode, various metals, such as iron, steel, nickel and Monel metal, can be employed on a laboratory scale, but iron is usually used in a commercial-scale electrolysis from the viewpoint of availability and economy. The electrolysis is generally conducted under conditions such that the current density is 7 to 13 A/dm.sup.2 and the bath voltage is 8.5 to 15 V.
The anode and cathode reactions which should occur in the electrolysis using the above method can be represented by the following formulae (1) and (2), respectively: R1 ? ? ##STR1##
It is known that when a carbon electrode is used as an anode in the electrolytic production of fluorine, the carbon electrode suffers the following serious problems (a), (b) and (c):
(a) One end portion of a carbon electrode, which is usually fixedly connected to a positive terminal for flowing an electric current to the anode in an electrolytic apparatus by means of a copper bolt and a copper nut, is likely to be largely destroyed at this portion of connection during the electrolysis.
(b) The mechanical strength of a porous carbon electrode is generally low, so that local breakage and gradual, partial coming-off of the carbon electrode are likely to occur during the electrolysis, even at portions other than the above-mentioned portion of connection, thereby producing fine particles of carbon. (Herein, "gradual, partial coming-off" means gradual, partial loss of a carbon electrode as carbon particles broken from the almost entire surface thereof.) These fine particles of carbon easily react with fluorine to thereby form CF.sub.4, and the resultant CF.sub.4 is disadvantageously contained in the fluorine as the desired electrolysis product.
(c) Due to the reaction between the carbon anode and F.sub.2 evolved at the carbon anode, a film of graphite fluoride having an extremely low surface energy is formed on the carbon electrode to cover the electrode. The wettability of the carbon electrode for the electrolytic bath is decreased at portions where graphite fluoride has been formed, so that the carbon electrode becomes electrochemically inactive at these graphite fluoride-covered portions. The effective surface area of the carbon electrode is decreased in accordance with the increase in the graphite fluoride-coverage ratio of the surface of the carbon electrode, and thus, the true current density on the carbon electrode is increased. This is the main cause of the anodic overvoltage observed in the electrolytic production of fluorine, and when the graphite fluoride-coverage of the carbon electrode exceeds 20% of the surface area, an abrupt, spontaneous rise of voltage is observed and it becomes no longer possible to flow an electric current through the carbon electrode. This phenomenon, which is known as the "anode effect", is a great problem encountered in commercially conducting the electrolysis of an HF-containing molten salt.
Among the above-described problems (a), (b) and (c), problem (c) has already been successfully solved by the present inventors by developing a method in which a metal fluoride mixture containing LiF is effectively introduced into the pores of a carbon block by skillful impregnation, thereby suppressing the occurrence of the anode effect during the electrolysis (see European Patent Application Publication No. 0 354 057).
However, the above-mentioned problems (a) and (b) (that is, destruction of the carbon electrode at its portion connected to the positive terminal for flowing an electric current to the anode as well as local breakage and gradual, partial coming-off of the carbon electrode) have not yet been solved, and have been of extreme seriousness in conducting the electrolysis of an HF-containing molten salt on a commercial scale. Therefore, development of a carbon electrode which is free from the above problems so that the electrolysis of an HF-containing molten salt can be stably performed for a prolonged period of time while assuring a high purity of a desired electrolysis product, has been earnestly desired.
In general, a carbon electrode comprises a porous carbon block which is prepared by a method in which coke, such as petroleum coke and pitch coke, is pulverized to prepare a base material and the base material is then blended with a binder, such as a coal-tar pitch and a synthetic resin, and the resultant blend is subjected to kneading, molding and heat treatment. The coke to be used in the above method as the base material has regions in which the crystallites of graphite are oriented in a certain direction at least to some degree. These crystallites of graphite grow and develop when the temperature is increased for heat treatment.
As a result of the intensive studies of the present inventors, it has been found that not only does a lower mechanical strength, such as a lower flexural strength, of a carbon electrode cause local breakage and gradual, partial coming-off of the carbon electrode, the chemical behavior, which is exhibited during the electrolysis of an HF-containing molten salt, of the above-mentioned graphite structure regions of the carbon electrode has close connection with the destruction of a portion of the carbon electrode where the carbon electrode is fixedly connected to the positive terminal which is positioned above the level of the electrolytic bath. That is, the present inventors have unexpectedly found that when a carbon electrode is exposed to an F.sub.2 atmosphere entraining HF, an intercalation compound is likely to be formed by a reaction represented by formula (3) shown below: ##STR2## and that due to the formation of the intercalation compound, the interlayer spacings of the graphite structure are widened to expand the carbon electrode, leading to a destruction of the carbon electrode.